In an image forming apparatus, such as an electronic copier, utilizing digital techniques, a document glass with a document placed thereon is exposed to light and the reflected light from the document glass is guided onto a CCD. The CCD outputs an image signal corresponding to the image on the document. A laser beam corresponding to the image signal illuminates a photosensitive drum to form an electrostatic latent image on the peripheral surface of the photosensitive drum. The electrostatic latent image is deposited with a developing agent (toner) to be converted to a visible image. A sheet of paper is sent to the photosensitive drum at a timing meeting the rotation of the photosensitive drum and the visible image (developing agent image) on the photosensitive drum is transferred to the sheet of paper. The sheet of paper with the developing agent image transferred thereto is sent to an image fixing apparatus.
The image fixing apparatus has a heating roller and a pressure applying roller set in pressure contact with the heating roller. These rollers sandwich the image-bearing sheet of paper and, while conveying that sheet of paper, fix the developing agent image on the sheet of paper under the heat of the heating roller to the sheet of paper.
As one example of a heating source of such a heating roller there is an induction-heating device. The induction heating device has a coil held within the heating roller and a high-frequency generating circuit for supplying a high-frequency current.
The high-frequency generation circuit outputs a high-frequency magnetic field. This high-frequency magnetic field is applied to the heating roller to create an eddy current in the heating roller. Self-generation of heat by the heating roller occurs as a result of that eddy current loss and a developing agent image on a sheet of paper is fixed by the heat generation to the sheet of paper.
In the case of an image fixing apparatus using the induction-heating device, the direction of the high-frequency magnetic field generated by the coil is constant at all times. In the case where the rotation of the heating roller is stopped as at starting time, ready time, etc., of the image forming apparatus, a varying temperature is created at a plurality of places along the peripheral surface of the heating roller. This varying temperature adversely affects the result of image fixing at copying time.
An image fixing apparatus according to a first aspect of the present invention is directed to eliminating temperature variation on the surface of a heating roller.
An image forming apparatus according to the first aspect of the present invention comprises a heating roller and a coil rotatably set within the heating roller to impart a high-frequency magnetic field to the heating roller.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.